Calendar date

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A calendar date is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "25 April 2024" is ten days after "15 April 2024". The date of a particular event depends on the observed time zone. For example, the air attack on Pearl Harbor that began at 7:48 a.m. Hawaiian time on 7 December 1941 took place at 3:18 a.m. Japan Standard Time, 8 December in Japan.

A particular day may be assigned a different nominal date according to the calendar used, so an identifying suffix may be needed where ambiguity may arise.^[a] The Gregorian calendar is the world's most widely used civil calendar,^[1] and is designated (in English) as AD or CE. Many cultures use religious or regnal calendars such as the Gregorian (Western Christendom, AD), Hebrew calendar (Judaism, AM), the Hijri calendars (Islam, AH), Julian calendar (Eastern Christendom, AD) or any other of the many calendars used around the world. In most calendar systems, the date consists of three parts: the (numbered) day of the month, the month, and the (numbered) year. There may also be additional parts, such as the day of the week. Years are usually counted from a particular starting point, usually called the epoch, with era referring to the span of time since that epoch.^[b]

A date without the year may also be referred to as a date or calendar date (such as "25 April" rather than "25 April 2024"). As such, it is either shorthand for the current year or it defines the day of an annual event, such as a birthday on 31 May, a holiday on 1 September, or Christmas on 25 December.

Many computer systems internally store points in time in Unix time format or some other system time format. The date (Unix) command—internally using the C date and time functions—can be used to convert that internal representation of a point in time to most of the date representations shown here.

Date format

There is a large variety of formats for dates in use, which differ in the order of date components. These variations use the sample date of 31 May 2006: (e.g. 31/05/2006, 05/31/2006, 2006/05/31), component separators (e.g. 31.05.2006, 31/05/2006, 31-05-2006), whether leading zeros are included (e.g. 31/5/2006 vs. 31/05/2006), whether all four digits of the year are written (e.g., 31.05.2006 vs. 31.05.06), and whether the month is represented in Arabic or Roman numerals or by name (e.g. 31.05.2006, 31.V.2006 vs. 31 May 2006).

Gregorian, day–month–year (DMY)

This

• "9 November 2006" or "9. November 2006" (the latter is common in German-speaking regions)
• 9/11/2006 or 09/11/2006
• 09.11.2006 or 9.11.2006
• 9. 11. 2006
• 9-11-2006 or 09-11-2006
• 09-Nov-2006
• 09Nov06 – Used, including in the U.S., where space needs to be saved by skipping punctuation (often seen on the dateline of Internet news articles).
• [The] 9th [of] November 2006 – 'The' and 'of' are often spoken but generally omitted in all but the most formal writing such as legal documents.
• 09/Nov/2006 – used in the Common Log Format
• Thursday, 9 November 2006
• 9/xi/06, 9.xi.06, 9-xi.06, 9/xi-06, 9.XI.2006, 9. XI. 2006 or 9 XI 2006 (using the
  bilingual form of the month. It was also commonly used in the Soviet Union
  , in both handwriting and print.
• 9 November 2006 CE or 9 November 2006 AD

Gregorian, year–month–day (YMD)

In this format, the most significant data item is written before lesser data items i.e. the year before the month before the day. It is consistent with the big-endianness of the Hindu–Arabic numeral system, which progresses from the highest to the lowest order magnitude. That is, using this format textual orderings and chronological orderings are identical. This form is standard in East Asia, Iran, Lithuania, Hungary, and Sweden; and some other countries to a limited extent.

Examples for the 9th of November 2003:

• 2003-11-09: the standard Internet date/time format,
  East Asian countries, as well as in some European countries. The big-endian convention is also frequently used in Canada, but all three conventions are used there (both endians and the American MMDDYYYY format are allowed on Canadian bank cheques provided that the layout of the cheque makes it clear which style is to be used).^[3]
• 2003 November 9
• 2003Nov9 or 2003Nov09
• 2003-Nov-9 or 2003-Nov-09
• 2003-Nov-9, Sunday
• 2003. november 9. – The official format in Hungary, point after year and day, month name with small initial. Following shorter formats also can be used: 2003. nov. 9., 2003. 11. 9., 2003. XI. 9.
• 2003.11.9 using dots and no leading zeros, common in China.
• 2003.11.09
• 2003/11/09 using slashes and leading zeros, common in Japan on the Internet.
• 2003/11/9
• 03/11/09
• 20031109 : the "basic format" profile of ISO 8601, an 8-digit number providing
  monotonic date codes, common in computing and increasingly used in dated computer file names. It is used in the standard iCalendar
  file format defined in RFC 5545. A big advantage of the ISO 8601 "basic format" is that a simple textual sort is equivalent to a sort by date.

This sequence is used primarily in the

The modern convention is to avoid using the ordinal (th, st, rd, nd) form of numbers when the day follows the month (July 4 or July 4, 2006).^[citation needed] The ordinal was common in the past and is still sometimes used ([the] 4th [of] July or July 4th).

Gregorian, year–day–month (YDM)

This date format is used in Kazakhstan, Latvia, Nepal, and Turkmenistan. According to the official rules of documenting dates by governmental authorities,^[5] the long date format in Kazakh is written in the year–day–month order, e.g. 2006 5 April (Kazakh: 2006 жылғы 05 сәуір).

Standards

There are several standards that specify date formats:

• ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times specifies YYYY-MM-DD (the separators are optional, but only hyphens are allowed to be used), where all values are fixed length numeric, but also allows YYYY-DDD, where DDD is the ordinal number of the day within the year, e.g. 2001–365.^[6]
• RFC 3339 Date and Time on the Internet: Timestamps specifies YYYY-MM-DD, i.e. a particular subset of the options allowed by ISO 8601.^[7]
• RFC 5322 Internet Message Format specifies day month year where day is one or two digits, month is a three letter month abbreviation, and year is four digits.^[8]

Many numerical forms can create confusion when used in international correspondence, particularly when abbreviating the year to its final two digits, with no context. For example, "07/08/06" could refer to either 7 August 2006 or July 8, 2006 (or 1906, or the sixth year of any century), or 2007 August 6, and even in some extremely rare cases^{[dubious – discuss]} it could mean 2007 8 June.

The date format of YYYY-MM-DD in ISO 8601, as well as other international standards, have been adopted for many applications for reasons including reducing transnational ambiguity and simplifying machine processing.^{[citation needed]}

An early U.S.

When transitioning from one calendar or date notation to another, a format that includes both styles may be developed; for example Old Style and New Style dates in the transition from the Julian to the Gregorian calendar.^[10]

Advantages for ordering in sequence

One of the advantages of using the

2003-02-28 (28 February 2003) sorts before
2006-03-01 (1 March 2006) which sorts before
2015-01-30 (30 January 2015)

The YYYY-MM-DD layout is the only common format that can provide this.[11] Sorting other date representations involves some parsing of the date strings. This also works when a time in 24-hour format is included after the date, as long as all times are understood to be in the same time zone.

ISO 8601 is used widely where concise, human-readable yet easily computable and unambiguous dates are required, although many applications store dates internally as

Specialized usage

Day and year only

The U.S. military sometimes uses a system, which they call "Julian date format"^[12] that indicates the year and the actual day out of the 365 days of the year (and thus a designation of the month would not be needed). For example, "11 December 1999" can be written in some contexts as "1999345" or "99345", for the 345th day of 1999.^[13] This system is most often used in US military logistics since it simplifies the process of calculating estimated shipping and arrival dates. For example: say a tank engine takes an estimated 35 days to ship by sea from the US to South Korea. If the engine is sent on 06104 (Friday, 14 April 2006), it should arrive on 06139 (Friday, 19 May). Outside of the US military and some US government agencies, including the Internal Revenue Service, this format is usually referred to as "ordinal date", rather than "Julian date".^[14]

Such ordinal date formats are also used by many computer programs (especially those for mainframe systems). Using a three-digit

Companies in Europe often use year, week number, and day for planning purposes. So, for example, an event in a project can happen on w43 (week 43) or w43-1 (Monday, week 43) or, if the year needs to be indicated, on w0643 (the year 2006, week 43; i.e., Monday 23 October–Sunday 29 October 2006).

An

Expressing dates in spoken English

In English-language outside North America (mostly in Anglophone Europe and some countries in Australasia), full dates are written as 7 December 1941 (or 7th December 1941) and spoken as "the seventh of December, nineteen forty-one" (exceedingly common usage of "the" and "of"), with the occasional usage of December 7, 1941 ("December the seventh, nineteen forty-one"). In common with most continental European usage, however, all-numeric dates are invariably ordered dd/mm/yyyy.

In

• Calendar algorithms
• Date and time representation by country
• Date and time notation in the United Kingdom
• Date and time notation in the United States
• Internationalization and localization
• ISO 8601 – an international standard covering the representation of dates and times
• List of calendars
• Time formatting and storage bugs
• Year 10,000 problem

Notes

References